1. Technical Field of the Invention
The present invention relates generally to tire inflation pressure detecting apparatuses for detecting the inflation pressures of tires on a vehicle.
More particularly, the invention relates to a direct-type tire inflation pressure detecting apparatus which has a capability of selecting a target transceiver from a plurality of transceivers and triggering only the target transceiver to perform a specific task.
2. Description of Related Art
A conventional direct-type tire inflation pressure detecting apparatus includes a plurality of transmitters and a receiver. Each of the transmitters is directly mounted on one of a plurality of wheels of a vehicle and includes a pressure sensor for sensing the inflation pressure of a tire fitted on the wheel. Each of the transmitters is configured to transmit a pressure signal representative of the inflation pressure of the tire sensed by the pressure sensor. The receiver is mounted on the body of the vehicle and includes at least one antenna. The receiver is configured to receive, via the antenna, the pressure signals transmitted by the transmitters and determine the inflation pressures of the tires based on the received pressure signals.
However, with the above configuration, it is impossible for the receiver to detect the locations of the transmitters. In other words, the receiver cannot identify the wheel on which the transmitter having transmitted a pressure signal is located.
To solve the above problem, Japanese Patent First Publication No. 2007-15491, an English equivalent of which is US Patent Application Publication No. 2007/0008097 A1, discloses a tire inflation pressure detecting apparatus which has a function of wheel identification. More specifically, this apparatus includes a plurality of transceivers each of which is mounted on one of a plurality of wheels of a vehicle, at least one triggering device that is mounted on the body of the vehicle at different distances from the transceivers, and a receiver located on the body of the vehicle. The triggering device transmits a trigger signal at a low frequency in the range of, for example, 125 to 135 kHz. The strength of the trigger signal attenuates with increase in the distance from the triggering device. Therefore, the strengths of the trigger signal at the transceivers are different from each other. In response to receipt of the trigger signal, each of the transceivers determines the strength of the trigger signal thereat and transmits a response signal which contains signal strength information about the determined strength of the trigger signal. The receiver receives the response signals transmitted by the transceivers and identifies, for each of the received response signals, the wheel on which the transceiver having transmitted the response signal is located based on the signal strength information contained in the response signal.
For such a tire inflation pressure detecting apparatus as described above, each of the transceivers may be configured to periodically transmit a pressure signal to the receiver. In this case, during inspection of the tire inflation pressure detecting apparatus before shipment, it is necessary for an inspector to transmit, using a special triggering tool, the trigger signal which contains a stop command, thereby triggering a target transceiver (i.e., a selected one of the transceivers) to stop the periodic transmission of the pressure signal.
However, when the distance between the triggering tool and the target transceiver is long, other transceivers than the target transceiver may also receive the trigger signal, thus being undesirably triggered to stop the periodic transmission before being inspected.
To avoid the above problem, two methods can be considered. One method is to lower all the receiver sensitivities of the transceivers and have the inspector transmit the trigger signal at a sufficiently short distance (for example 30 cm) from the target transceiver. The other method is to lower the strength at which the trigger signal is transmitted and have the inspector transmit the trigger signal at a sufficiently short distance from the target transceiver. However, with either of the two methods, the inspector has to locate the triggering tool in close vicinity to the target transceiver, which is generally fixed to the air valve of the associated wheel, thus significantly lowering inspection efficiency.
Furthermore, during operation of the tire inflation pressure detecting apparatus, it is desirable for the triggering device to transmit the trigger signal which contains a specific command, thereby triggering only a target transceiver (i.e., a selected one of the transceivers) to perform a specific task.